On page 55 of "Bad Astronomy" Mr. Plait says that it is important for the Hubble Space Telescope to figure in the precession of the earth axis. Why is this if the telescope is out in space and not on the earth?

The coordinate system against which celestial objects are measured (right ascension/declination) is anchored to the Earth's rotation, and therefore the direction of the celestial poles change with the Earth's precession. At the very least, any accurately pointed observation (on Earth or off) has to take into account the epoch of the coordinate system used as input (typically these days, 2000.0) and the precession from then to the observed time. In principle one could work straight from some well-defined inertial coordinate system, but systems anchored to the Earths axis remain the most accurately defined (AFAIK).

I understand the need for precession to be accounted for when observing from the earth. Your statement does not explain why this is necessary in space. I would think that if you are orbiting Jupiter you do not need to take earth's precession into consideration when locating an object. I would think a co-ordinate system separate from the one used for ground based telescopes would be better. The only reason I can think of to use the ground based system would be that the Hubble Telescope's orbit is somehow linked to the earth's axis and through (gravity?) is preturbed to mimic the precession of earth's axis.

I hope I can possibly answer your question---although it has been several years since I read the book-- there are probably a number of reasons---

the first of which may be: we have (for the most part) been building fewer ground-based optical-type telescopes for some time now ... because they eventually are rendered too costly because of light and particle pollution. The most notable exceptions are those that on the Hawaiian islands and in a few countries in South America. Also the atmosphere of the earth is impenetrable to many types of electromagnetic radiation--so it seemed logical to put telescopes into orbit

the second of which may be: Servicing of the telescopes might be easier if they were put into predictable orbits.

the third of which may be: the rendering of post-image processing might serve to cancel any propagation of error that may occur in the production of the image(?)--that is my logical guess?

I hope someone else can lend a hand here----

Cheers and Welcome to the forum...

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"The candle flame gutters. Its little pool of light trembles. Darkness gathers. The demons begin to stir."
---Carl Sagan

The recorded positions of celestial objects are given in a coordinate system that has a relation to the Earth. The positions of stars change in this system due to (among other things) precession of the equinoxes. Stored in the HST’s computer’s memory are the positions of stars at 2000.0 in that Earth based coordinate system. On other dates, the predicted position of the stars must be adjusted to account for (among other things) precession of the equinoxes. Indeed, if the positions of the stars had been initially recorded in an inertial coordinate system, and if the HST used that system, then an adjustment for precession would not have been necessary.

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Curt Renz - "Centaur"
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